Voltage and light induced strains in porous crystalline materials and uses thereof

ABSTRACT

A piezoelectric device is disclosed, which includes a first element of porous crystalline material, a second element being attached to, or integrally formed with, the first element, and at least one electrode being in electrical contact with the first element, such that subjecting the first element to an electric potential via the at least one electrode results in a strain induced by the first element on the second element. Also disclosed is a piezooptic device which includes a first element of porous crystalline material, a second element being attached to, or integrally formed with, the first element, and a light source, such that subjecting the first element to light originating from the light source results in a strain induced by the first element on the second element.

[0001] This is a divisional of U.S. patent application Ser. No.09/613,759, filed Jul. 11, 2000.

FIELD AND BACKGROUND OF THE INVENTION

[0002] The present invention is derived from a striking novel discovery,that porous crystalline materials have piezoelectric and piezoopticproperties. The present invention, therefore, relates to devices andmethods which take advantage of this newly discovered phenomenon. Onemain use of this discovery is in the field of adaptive optics, e.g.,adaptive reflectors such as mirrors, however, other uses, as is furtherdelineated below, are anticipated. Thus, for example, the new technologycan find uses in (i) attached fiber optics maneuvering (movement offiber for focusing, etc.); (ii) attached fiber optic bending for modecontrol; (iii) attached fiber optics intensity and polarization control;(iv) spatial light modulation (electro optical and opto-opticalmodulation) by adjustment of specific elements or a whole device; (v)tunneling devices, where the tunneling current is sensitive to distancebetween elements; (vi) scanning microscopy heads, optical or magneticdisc readers which have to be maneuvered by electric or light signal;and (vii) light or voltage detectors.

[0003] Most of the following background discussion, however, focuses onthe construction, fabrication, use, advantages and limitations of priorart adaptive mirrors, however, there is no intention to restrict the useof the newly discovered piezoelectric and piezooptic properties ofporous crystalline materials to the field of adaptive optics, as manyother uses and applications of this core technology are envisaged.

[0004] Adaptive optics systems are essentially a servo loop, with asensitive wave front sensor, a control computer, and a flexible mirrorto correct aberrations in a beam of light. Despite large efforts made inthe last few decades, progress in deformable mirrors has been slow, andthere are only a few kinds available. The high price of these mirrors isan indicator of the problems in their manufacture, such as complexconstruction, non-repeatability and non-uniformity.

[0005] What is required of an adaptive mirror? It has to be agile enoughto correct even the strongest and densest wave front fluctuations(usually a few micrometers in stroke), while not contributing errors ofits own. The more elements it has the better, ranging between few andthousands of actuators. It has to be quick enough to correct even thefastest variations, while not resonating close to the operationalfrequency. It has to run on low power to avoid a cumbersome power supplyand control system, while not loosing in dynamic range. It has to besmall and light enough to mount in a compact space. It has to befail-safe or at least allow easy correction or replacement of badelements. It has to have serial command lines to the elements to avoidmassively parallel wiring. Mechanically, it needs to be of good opticalquality and insensitive to temperature variations, even without activecorrection. Finally, it ought to have a sound price, which is derivedmainly from its construction technology.

[0006] There are many other fields where actuation of devices is a partof their operation: communication devices, switching devices, scanningmicroscopes, printers, and many more depend on mechanical movement ofsmaller or larger parts as a response to an electronic or opticalcommand. The discussion below will concentrate on adaptive optics (orthe slower active optics) as a relevant example: such systems change thepath of light beams, their direction or the wave front emanating fromthem, usually to correct aberrations.

[0007] Lets start with the simpler systems, those that can serve asbasis for systems that are more complex. Lets define these systems asbeing able to correct only one mode at a time. The lowest modes would bethose which can be defined by one parameter over the correction area.Zernike decomposition, which is common for optical round apertures, hasbasic modes as follows: (i) piston correction (given value of the wavefront), this mode is important only when using a segmented mirror; (ii)tip and tilt (given value of the wave front derivatives in x and ydirections; and (iii) defocus (given value of the wave front curvature).

[0008] Piston correction is achieved merely by moving the mirror surfaceup and down, while maintaining its direction. The size of the elements dis small compared to the lateral scale of the aberrations. Mirrormovement in parallel to itself is achieved by piston actuators; theseactuators are the basis of most deformable mirrors. Mentioned here aresome commonly used devices. Comparative designs and analyses haveappeared in J. A. Pearson, R. H. Freeman, and Harold C. Reynolds, Jr.,‘Adaptive optical techniques for wave-front correction’, in AppliedOptics and Optical Engineering Vol. VII, R. R. Shannon and. C. Wyant,editors, Academic Press, 246-340, 1979; M. A. Ealey, ‘Actuators: designfundamentals, key performance specifications, and parametric trades’,SPIE Vol. 1543, 346-362, 1991; M. A. Ealey and J. A. Wellman,‘Deformable mirrors: design fundamentals, key performancespecifications, and parametric trades’, SPIE 1543, 36-51, 1991; E. N.Ribak, ‘Deformable mirrors’, in Adaptive Optics in Astronomy, NATO ASIVol. 423, 149-62, 1994; R. K. Tyson, Principles of Adaptive Optics,Academic Press, 1998; R. E. Aldrich, ‘Deformable mirror wavefrontcorrectors’, in Adaptive Optics Engineering Handbook, Marcel Dekker,2000. The main types of deformable mirrors appear in FIGS. 1a-f, each ofwhich has its limitations.

[0009] The most common method of piston correction is by usingpiezoelectric actuators. These actuators are very convenient since theyrespond directly and quite linearly to an applied voltage. Theirresponse (for lead zirconium titanate, the most common material) is inthe order of 1 micrometer for 1 kV, which is too small to achieve theseveral microns required to correct for atmospheric turbulence. Only onemirror was used in this configuration: the monolithic piezoelectricmirror[J. S. Feinlieb, S. G. Lipson, and P. E. Cone, ‘Monolithicpiezoelectric for wavefront correction’, Appl. Phys. Lett. Vol. 25,311-315, 1974], where the electrode contacts were drilled through theactuator block almost to the face of the mirror. A number of schemeswere devised for better voltage response. In one scheme use is made ofthe thickness-to-length ratio: instead of applying the voltage along themost responsive direction, it is applied across this direction. Bymaking the piezoelectric material very long and very thin, thetransverse response is amplified by this ratio. Since they areconstructed from ceramic materials, these actuators can be manufacturedin almost any desired shape. For this application they are prepared in atubular shape, which is convenient for other applications. Anotherchoice is to bond a stack of many thin slabs of the material, and applylow voltage on all of them in cascade. The slabs are combined with theirpolarizations directions alternating so that application of the voltageis in parallel.

[0010] Another piezoelectric material is lead magnesium niobate, with abetter (but uni-directional) voltage response and lower hysteresis [G.H. Blackwood, P. A. Davis, and M. A. Ealey, ‘Characterization of MMN:BAelectrostrictive plates and SELECT actuators at low temperatures’, SPIEVol. 1543, 422-429, [Blackwood et al., 1991]. This kind of response iscalled electrostrictive. However, since it always extends, either underpositive or negative voltage, a bias voltage must be applied to it forbidirectional movement.

[0011] Another option for actuators is the voice coil, such as used incommercial loud speakers. In this case, a magnetic field drives a coilattached to a piston. The drawback here is the heating created by theflowing current. The magnetostrictive actuator consists of a solenoidwithin which is the magnetostrictive ferrite whose length changes undermagnetic field. Here again the actuator has to be rid of the heat in thesolenoid. It is easier to use these actuators in a laser adaptivemirror, since both have to be cooled anyway.

[0012] The hydraulic actuator is able to provide force by amplificationof mechanical force or by employing a valve to control a constant highpressure. Severe drawbacks such as requirements for both a hydraulicsystem and an electrical one, slow response and large volume make themless convenient to employ.

[0013] Finally, electrostatic actuation is used for membrane mirrors,and is discussed further below. A full comparison between the differentactuators can be found in the general references cited above.

[0014] Tip and tilt correction should be separated from the case of asteering correction. Tip and tilt are required in multi-element systems,where the wave front will not be continuous between actuators. Tominimize this effect the tip and tilt are supplemented by pistonmovement.

[0015] A steering mirror only corrects the direction of the incomingbeam. It is utilized for telescopes which cannot track in a smoothmanner or for initial correction for turbulence-induced wave fronttilts. In this capacity it is also employed sometimes as the first stagein an adaptive optics system to reduce requirements on the maindeformable mirror. A full list of design parameters for such mirrors canbe found in [L. M. Germann, ‘Specifications of fine-steering mirrors forline-of-sight stabilization systems’, SPIE Vol. 1543, 202-212, 1991].

[0016] Steering mirrors have two degrees of freedom, whereas tip-tiltcorrectors require three. All designs on the market today utilizepistons to push and pull on the back of a high quality mirror. Some ofthe mirrors are metallic (molybdenum or beryllium) for powerapplications and for high speed response. The requirement to maintain aflat surface is alleviated if steering mirrors are used as a first stagebefore a deformable mirror, since a servo loop could correct for theresidual errors.

[0017] Steering and tip/tilt/piston mirrors can have a number ofmechanical designs. One design uses direct piezoelectric actuators orlever-amplified ones. In another design use is made of tubularpiezoelectric material. Sectors along the tube are powered separately toboth bend and piston the supported mirror. There are also voice coilpistons and electromagnetic pistons. The number of actuators variesbetween two and four, depending on the specific application.

[0018] Defocus can be corrected by a mirror whose radius of curvaturecan be controlled. This mode of operation can be achieved in the bimorphmirror. In this device, the actuator is not pushing against the back ofthe mirror or pulling it down, but acts to stretch along the surface ofthe mirror. In a construction similar to the bimetallic strip, a thinactuator is glued to the surface of a thin mirror. When voltage isapplied to it, it expands in area (when thin enough, the lateralcontraction is negligible). In a manner similar to the bimetallic stripunder heating, this expansion, combined with the inert mirror, leads tothe structure curving. Another possibility is to have the actuatorsglued back to back so that the bending is doubled. In this case theyhave to be polished properly to optical quality. It can be shown[Steinhaus E., and S. G. Lipson, ‘Bimorph piezoelectric flexiblemirror’, J. Opt. Soc. Am. Vol. 69, 478-481, 1979] that the curvature ofthe surface is proportional to the applied voltage.

[0019] Another way to achieve a spherical surface is by electrostaticpull. A thin conducting membrane is pulled towards a plane and(initially) parallel electrode which is charged to create a capacitor.The amount of charge sets the amount of curvature. A transparentelectrode is sometimes required to pull on the membrane in the oppositedirection and to protect it from acoustic noise sources[F. Merkle, K.Freischlad, J. Bille, ‘Development of an active optical mirror forastronomical applications’, ESO Conference on Scientific Importance ofHigh Angular Resolution at Infrared and Optical Wavelengths, 41-44,1981, M. Clampin, S. T. Durrance, D. A. Golimowski, and R. H.Barkhouser, ‘The Johns Hopkins adaptive optics coronograph’, SPIE Vol.1542, 165-74, 1991, G. Vdovin, ‘Micromachined membrane deformablemirror’, in Adaptive Optics Engineering Handbook, Rk. Tyson, Rd., MarcelDekker 2000].

[0020] So far the discussion was focused on segmented mirrors which aresimpler to construct and maintain, easily understood, andstraightforward to run. Unfortunately, they cannot mimic the aberratedwave front too well, and they have gaps between the mirrors.

[0021] These drawbacks do not exist for the competition: continuousmirrors. Here the front surface consists of a single unit, usuallycalled the face plate or face sheet, and it is acted upon from behind byactuators of various sorts. The actuators separate into two kinds:piston actuators, which act on the face sheet normal to its surface, andbending actuators, which act in parallel to the surface.

[0022] Piston actuation: Applying a force normal to the surface requiresa special attachment of the actuator to the face plate. The actuator hasto push and pull on the surface using some heavy and solid backreference plane. The attachments of the actuator to the base and to themirror have some special—and sometimes conflicting—requirements:

[0023] 1. Yield or backlash below the required wave front accuracy(usually below 0.1 micrometer in the normal direction, 0.5 micrometerinside the plane of the face sheet).

[0024] 2. Possibility for simple and accurate replacement of a faultyactuator (this is especially important for multiple element mirrors).

[0025] 3. Foot-print (projected area of the actuator on the mirror)allowing high density of elements. The base attachment foot-print alsohas to allow power lines to the actuator.

[0026] 4. Print-through (induced local mirror aberration) at therequired wave front accuracy (say 0.1 micrometer) at all lateral scaleswhich cannot be corrected by the actuators themselves.

[0027] 5. Adjustment for zero power: means to set the mirror surfaceflat if the actuators are not powered (the telescope must still functionwhen the deformable mirror is turned off). In some cases this adjustmentcan be achieved by a constant bias on the elements, which reduces thedynamic range of the actuators. In these cases the zero adjustment canbe coarser and only place the actuator in its application margin.

[0028] 6. Preloading of the actuators is sometimes required.Piezoelectric actuators, for example, function better against pressure.

[0029] 7. The three-dimensional shape of the attachment is extremelyimportant for tailoring an appropriate influence function.

[0030] 8. The frequency response of the structure has to be as linear aspossible, and the first resonance frequency high above the atmosphericfastest fluctuations.

[0031] A common attachment between actuator and face plate is magnetic.A thin ferromagnetic plate is glued to the back of the face plate.Permanent magnets are used to attach the actuators to it. Another choiceis to glue an end piece to the mirror and to attach the actuator to it.In less expensive mirrors the actuator is glued directly to the faceplate.

[0032] Influence functions: When an actuator pushes or pulls on amirror, the surface of the mirror attains a hill or a valley shapecentered around the attachment point. This shape is called the actuatorinfluence function, although analytic functions do not describe it toowell. It depends on the following parameters:

[0033] 1. Face plate material properties.

[0034] 2. Actuator attachment geometry and material.

[0035] 3. Location of neighboring actuators. The distance to theneighbors, their arrangement (square or hexagonal), and even thenon-existence of neighbors near the border all affect the influencefunction.

[0036] 4. Repeatability of the response of the actuators and theirattachments.

[0037] The importance of the influence function is in the calculation ofthe fit of the mirror to the wave front. If each actuator has a threedimensional influence function of its own, than the control computermust perform a very large and time consuming fit of the whole wave frontto the whole mirror. Even influence functions whose size is larger thanthe actuator-to-actuator distance complicate this calculation. Thus,having a constant and repeatable influence function which can betabulated or modeled by a simple function (e.g., cubic spline, gaussian,cosine) is extremely important. This difficult point was realized andtackled from very early on [see the general references and H. R.Hiddleston, D. David Lyman, and E. L. Schafer, ‘Comparisons ofdeformable mirror models and influence functions’, SPIE Vol. 1542,20-34, 1991]. The problem is much easier for membrane mirrors. The useof faster and larger computers has somewhat reduced the need for astationary influence function.

[0038] Membrane mirrors: Membrane mirrors are those whose face plate isrelatively thin. Deformable mirrors which use piston actuation tend tohave thicker face plate, since the available force is usually much morethan required (although the stroke might be limited). In two cases thereis an advantage to thinner surfaces. Both cases allow longer strokes,but are limited in force and have rather high acoustic pick-up. Thefirst case is the electrostatic membrane mirror, which belongs in thepiston actuators, and the second is the bimorph mirror, which utilizesbending of the membrane surface.

[0039] The electrostatic membrane deformable mirror is essentially madeof a set of capacitors which are laid in parallel, both physically andelectrically. The membrane is made of a conducting, reflective materialof limited thickness (a few microns). This device is the extension of asingle such mirror. The electrostatic pull between the surfaces of thecapacitor is uni-directional, and it is required to devise some schemeto have a two-directional motion. One way is to bias all the elements atsome high voltage, and have all elements move in a small range above andbelow this bias. This electric bias creates a spherical bias surfacewhich has to be included in the design of the optical system. A way toavoid bias is to put an opposite electrode on the other side of themembrane to pull in the other direction. This electrode is transparentand allows the enclosure of the whole device.

[0040] The voltages required to achieve the desired stroke depend on thespacing between the membrane and the opposite electrodes, and rangebetween few volts to hundreds of volts. This spacing cannot be made toosmall so as to avoid the membrane short-circuiting to the electrodes,and also to allow some air to remain and dampen vibrations of themembrane. Too tight a space is also a problem since the air has no roomto escape when the membrane moves. The small room between the electrodesand the membrane require very accurate machining of the electrodes toavoid edge effects (sharp edges have a higher electrostatic field). Abiased membrane will require a curved electrode surface to best matchits equilibrium position. Finally, the membrane tends to vibrate veryeasily, so it needs an efficient damping mechanism (such as air) to beincluded in the design.

[0041] Bimorph mirrors: Bimorph mirrors depend on membrane face sheetsfor flexibility. They are constructed of a thin piezoelectric materialbonded to a thin mirror. The curvature depends on the square of ratio ofthe diameter to the thickness, which explains why they come under theheading of membranes. There are a number of ways to have a multipleelectrode bimorph mirror. The first is to have a large piezoelectricsheet attached to a large face sheet. The electrodes are drawn on theback of the piezoelectric sheet in any desired shape[F. Forbes and N.Roddier, ‘Adaptive optics using curvature sensing’, in SPIE Vol. 1542,140-147, 1991]. This method is limited due to the fragile nature of theceramic piezoelectric material that does not allow two high a ratio ofthe diameter to the thickness.

[0042] A second method is to glue many electrodes to the back of asingle face sheet[E. N. Ribak, S. G. Lipson, and C. Schwartz, ‘Thinmirror adaptation by simulated annealing’, ESO Conference onhigh-resolution imaging by interferometry II, 1991, ‘High performance,affordable agile mirror’, Air Force workshop on Declassification ofMilitary Technology: Laser Guide Stars, Albuquerque, N. Mex., 1992].This allows for the same piezoelectric material thickness to effectivelyincrease the size of the device. The draw-back is that a very thin facesheet is sensitive to print-through effects caused by edges of thepiezoelectric elements. Glue expansion at the elements edges is a severeproblem, which can only be resolved by using thicker face sheet andreducing the voltage sensitivity[C. Schwartz, E. Ribak, and S. G.Lipson, ‘Bimorph adaptive mirrors and curvature sensing’, J. Opt. Soc.Am. A Vol. 11, 895-902, 1994]. A stroke of one wave length requiresbetween seven and twenty volts, depending on the dimensions of thebimorph.

[0043] A significant difference between piston mirrors and membranemirrors is their voltage response. Each actuator has a spatial responsewhich is linear with its displacement and extends to approximately thenext element. Bimorph mirrors can be shown to solve the bi-harmonicequation, or, under simplifying assumptions, the Poisson equation[Steinhaus E., and S. G. Lipson, ‘Bimorph piezoelectric flexiblemirror’, J. Opt. Soc. Am. Vol. 69, 478-481, 1979; C. Schwartz, E. Ribak,and S. G. Lipson, ‘Bimorph adaptive mirrors and curvature sensing’, J.Opt. Soc. Am. A Vol. 11, 895-902, 1994]. This means that the surfacecurvature is linear with the voltage distribution, and that thisresponse extends to neighboring elements.

[0044] Because curvature is induced, not displacement, the membranemirror is easier to control using most wave front sensors. Essentiallyall such sensors measure either the wave front first derivative(Hartmann-Shack sensors, shearing interferometers) or the secondderivative (curvature sensors). In order to calculate displacement, itis necessary to integrate the gradient or laplacian measurements once ortwice. Differentiating the gradient or directly applying the curvatureto the membrane is much simpler[F. Forbes and N. Roddier, ‘Adaptiveoptics using curvature sensing’, in SPIE Vol. 1542, 140-147, 1991; C.Schwartz, E. Ribak, and S. G. Lipson, ‘Bimorph adaptive mirrors andcurvature sensing’, J. Opt. Soc. Am. A Vol. 11, 895-902, 1994]. Somemodification might be needed because the coupling between the curvaturesensor and the bimorph mirror has to take into account coupling betweencorrection terms and edge effects.

[0045] Addition of tip/tilt correction: The most severe aberration ofthe wave front is due to very large scale fluctuations. Since thedynamic range of the stroke of most piston mirrors is quite limited,this low order aberration is often taken care of separately by asteering mirror. The main deformable mirror corrects only residualerrors of higher frequency. The situation is better for bimorph mirrors,since their stroke is usually longer. In addition, correction of thelower Fourier components can be achieved on the border and outside theactive mirror surface, with virtually no effect on the rest of theelements. This is because the voltage sets the bimorph curvature, andthis curvature is independent of the large scale tilt.

[0046] Various other devices were proposed for deformable mirrors.Spatial light modulators, useful for image processing, are usuallylimited by spectral, spatial and temporal band-width. Oil films whosethickness can be varied electrostatically were also proposed in thepast, but rejected for the same reasons. Two more options are theutilization of laser corrective mirrors and of liquid crystalmodulators.

[0047] Corrective laser mirrors: Devices built for correction of lasermode hopping and for transmission of laser beams through turbulence canalso serve for astronomical applications. These mirrors are manufacturedto tolerate very high intensities, and the mirrors for atmosphericcorrection also respond at high enough frequency. The face plates areusually metallic (e.g., beryllium, molybdenum) for good thermalconductivity. The structure includes means for liquid cooling of thefront surface and sometimes of the actuators. Unfortunately, thesequalities make the mirrors extremely complex to construct, maintain, andrun.

[0048] Liquid crystals: Liquid crystals were proposed for wave frontcorrectors. The mechanism is electro-optical path correction bymodulation of nematic liquid crystals. An addressable matrix has therefractive index change by as much as 0.2 on a scale of 10 micrometers.Light reflected through the liquid crystal has its optical path changedat the rate of more than 1 micrometer in 50 ms, which is adequate. Avery large demagnification of the telescope aperture is required inorder to match it to this device. The technology seems to be maturingtowards its application for actual systems [G. D. Love, ‘Liquid crystaladaptive optics’, in Adaptive Optics Engineering Handbook, R. K. Tyson,Ed., Marcel Dekker, 2000].

[0049] System aspects: Here we deal with deformable mirrors as acomponent in a system. However, some considerations apply regarding thedeformable mirror as a subsystem in the adaptive optics system. This isimportant since the measurement and computation loads are very heavy.The deformable mirror has to be designed to relieve some of this load.

[0050] Adaptive optics systems can perform either zonal or modalcorrection. The first describes correction of local errors, whereas thelatter describes correction of modes of either the atmosphere or thetelescope. In this view it is possible to design the actuators to matchspecific modes [Clampin [F. Roddier, J. E. Graves, D. McKenna, and M.Northcott, ‘The University of Hawaii adaptive optics system’, SPIE Vol.1542 248-72, 1991; M. Clampin, S. T. Durrance, D. A. Golimowski, and R.H. Barkhouser, ‘The Johns Hopkins adaptive optics coronograph’, SPIEVol. 1542, 165-74, 1991]. The advent of fast processors and computershas simplified the systems even more. Instead of calculating specificmodes and applying appropriate commands, it is possible to either breakthe calculation into many parallel processors, where each controls itsown mode, or calculate in advance a transfer matrix between inputs andoutputs to be applied in every step.

[0051] Membrane mirrors, and to a lesser extent piston mirrors, aresensitive to vibrations. The design of the mirrors should be selected soas to maximize the first resonance frequency above typical atmosphericfrequencies. Otherwise it is essential to have the control and commandcircuits reduce the effects of these resonances.

[0052] A limiting factor in the design of deformable mirrors is theirdrivers or amplifiers. Most existing devices require either high voltageor high currents. Their drivers, running in parallel (one each for anactuator), a large volume and create a great amount of heat which has tobe disposed off away from the telescope. Thus they have to be isolatedfrom the telescope enclosure to avoid adding to the turbulence. Thiscontradicts the requirement that the transmission lines from theamplifiers to the mirrors should be as short as possible. Low-powersystems such as the liquid crystal and the bimorph mirror have an edgesince they can be run directly out of the controller without interveningamplifiers.

[0053] There is thus a widely recognized need for, and it would behighly advantageous to have, an adaptive mirror devoid of the abovelimitation. In addition, there is a need for other mechanical deviceswhich can help in maneouvering miniscule elements or devices, such asfiber optics, scanning microscope heads or memory devices reading andwriting heads.

SUMMARY OF THE INVENTION

[0054] According to one aspect of the present invention there isprovided a piezoelectric device comprising a first element of porouscrystalline material, a second element being attached to, or integrallyformed with, the first element, and at least one electrode being inelectrical contact with the first element, such that subjecting thefirst element to an electric potential via the at least one electroderesults in a strain induced by the first element on the second element.

[0055] According to another aspect of the present invention there isprovided a method of producing a piezoelectric device comprising thesteps of attaching to, or integrally forming with, a first element ofporous crystalline material, a second element, and attaching to thefirst element at least one electrode, such that subjecting the firstelement to an electric potential via the at least one electrode resultsin a strain induced by the first element on the second element.

[0056] According to yet another aspect of the present invention there isprovided a piezooptic device comprising a first element of porouscrystalline material, a second element being attached to, or integrallyformed with, the first element, and a light source, such that subjectingthe first element to light originating from the light source results ina strain induced by the first element on the second element.

[0057] According to still another aspect of the present invention thereis provided a method of producing a piezooptic device comprising thesteps of attaching to, or integrally forming with, a first element ofporous crystalline material, a second element, and providing at leastone light source, such that subjecting the first element to lightoriginating from the at least one light source results in a straininduced by the first element on the second element.

[0058] According to an additional aspect of the present invention thereis provided a method of inducing strain in a first element, the methodcomprising the steps of attaching to the first element, or integrallyforming with the first element, a second element of porous crystallinematerial and subjecting the second element to electric potential.

[0059] According to still an additional aspect of the present inventionthere is provided an adaptive reflector comprising a first layer ofporous crystalline material being attached to, or integrally formedwith, a second layer having a reflective surface.

[0060] According to further features in preferred embodiments of theinvention described below, the reflective surface is formed as areflective coat over the first layer.

[0061] According to still further features in the described preferredembodiments the reflective surface is designed to reflect ultravioletwaves.

[0062] According to still further features in the described preferredembodiments the reflective surface is designed to reflect light waves.

[0063] According to still further features in the described preferredembodiments the reflective surface is designed to reflect infraredwaves.

[0064] According to still further features in the described preferredembodiments the reflective surface is designed to reflect micro waves.

[0065] According to still further features in the described preferredembodiments the reflective surface is designed to reflect radio waves.

[0066] According to still further features in the described preferredembodiments the adaptive reflector further comprising at least oneelectrode through which an electric potential is applicable to the firstlayer.

[0067] According to still further features in the described preferredembodiments the adaptive reflector further comprising at least one lightsource with which light is applicable to the first layer.

[0068] According to still further features in the described preferredembodiments the porous crystalline material is porous silicon.

[0069] According to still further features in the described preferredembodiments the second element is made of a crystalline material, suchas crystal silicon.

[0070] According to a further aspect of the present invention there isprovided a method of straining a porous crystalline material element,the method comprising the step of subjecting the porous crystallinematerial element to electric potential.

[0071] According to still a further aspect of the present inventionthere is provided a method of straining a porous crystalline materialelement, the method comprising the step of subjecting the porouscrystalline material element to light.

[0072] According to yet a further aspect of the present invention thereis provided a method of relaxing a porous crystalline material elementwhich is subjected to an electric potential, the method comprising thestep of preventing the electric potential from the porous crystallinematerial element.

[0073] According to still a further aspect of the present inventionthere is provided a method of relaxing a porous crystalline materialelement which is subjected to light, the method comprising the step ofpreventing the light from the porous crystalline material element.

[0074] Implementation of the methods and devices of the presentinvention involves performing or completing selected tasks or stepsmanually, automatically, or a combination thereof. Moreover, accordingto actual instrumentation and equipment of preferred embodiments of themethods and devices of the present invention, several selected stepscould be implemented by hardware or by software on any operating systemof any firmware or a combination thereof. For example, as hardware,selected steps of the invention could be implemented as a chip or acircuit. As software, selected steps of the invention could beimplemented as a plurality of software instructions being executed by acomputer using any suitable operating system. In any case, selectedsteps of the methods and devices of the invention could be described asbeing performed by a data processor, such as a computing platform forexecuting a plurality of instructions. In addition, various strain andother sensors may be integrated in any of the devices of the presentinvention so as to monitor, feedback and control, via the specifiedhardware, the operation of a piezoelectric or piezooptic based devicesof the present invention. Such sensors are well known in the art.

[0075] The present invention successfully addresses the shortcomings ofthe presently known configurations by providing novel piezoelectric andpiezooptic devices and applications therefor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0076] The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin the cause of providing what is believed to be the most useful andreadily understood description of the principles and conceptual aspectsof the invention. In this, no attempt is made to show structural detailsof the invention in more detail than is necessary for a fundamentalunderstanding of the invention, the description taken with the drawingsmaking apparent to those skilled in the art how the several forms of theinvention may be embodied in practice.

[0077] In the drawings:

[0078]FIGS. 1a-f show simplified cross sections of several prior artdeformable mirrors: (a) piston activation of flat mirrors; (b) pistonactivation of a continuous mirror; (c) a piezoelectric monolithicmirror; (d) electrostatic membrane mirror; (e) bending moments usingpistons operable with either segmented mirrors or continuous mirrors;and (f) a bimorph mirror.

[0079]FIG. 2 is a schematic depiction of an optical measurement systembased on a curvature wave front sensor. A point source is collimated andreflected off a sample. A moving lens images two planes—the sample andone close to it—alternately on the camera. The off-sample plane exhibitsintensity changes as a function on voltage on the sample. All theseimages are grabbed into the computer and processed to yield the voltageresponse of the surface of the mirror.

[0080]FIG. 3 shows dependence of the silicon-porous silicon bimorphmirror on voltage. One sees the reconstructed wave front reflected offthe mirror, after subtraction of the wave front at zero volts. Themirror was slightly curved due to the manufacture process: the frontside of a 200 micrometer thick n-type 100-silicon wafer was evaporatedwith aluminum; the back side was etched in 50 % HF-ethanol=1:1 for 35minutes, at current density of 24 mA/cm² under 50 Watt halogenillumination. The porous layer created was 50 micrometers thick, and wasfurther passivated in oxygen at 450° C. for 30 minutes. Two wires wereattached by silver paint to the porous surface and one to the frontaluminized surface, serving as ground. The diameter of the wafer was 51mm, of the porous silicon 26 mm, and of each electrode 2 mm. The visiblesection shows 15 mm of the sample, with two electrodes, one at thebottom right of this section and one at the top left. Elevation unitsare in micrometers.

[0081]FIGS. 4a-b show typical dependence of the curvature (Zernikedefocus coefficient) of the wafer on the voltage for two samples. Thecurvature was measured over the whole wafer, and not near the electrodeswhere it peaks.

[0082]FIG. 5 is a simplified perspective view of a piezoelectric deviceaccording to the present invention.

[0083]FIG. 6 is a simplified perspective view of a piezooptic deviceaccording to the present invention.

[0084]FIG. 7 is a simplified perspective view of an adaptive reflectoraccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0085] The present invention is of piezoelectric and piezooptic devices,methods of their fabrication and uses thereof. Specifically, the presentinvention is of piezoelectric and piezooptic devices including porouscrystalline materials.

[0086] The principles and operation of the present invention may bebetter understood with reference to the drawings and accompanyingdescriptions.

[0087] Before explaining at least one embodiment of the invention indetail, it is to be understood that the invention is not limited in itsapplication to the details of construction and the arrangement of thecomponents set forth in the following description or illustrated in thedrawings. The invention is capable of other embodiments or of beingpracticed or carried out in various ways. Also, it is to be understoodthat the phraseology and terminology employed herein is for the purposeof description and should not be regarded as limiting.

[0088] In recent years crystalline silicon micromachining technology hasenabled the development of numerous miniaturized devices such aschemical reaction chambers, micro-actuators, micro-grippers,microvalves, etc. Also, it has been recently established thatminiaturized porous silicon structures can be fabricated usingmicromachining technology. To this end, see, for example, P. C. Searson,‘Porous Silicon Membranes’, Appl. Phys. Lett. Vol. 59, 1991; and S.Ottow et al., ‘Processing Of Three-Dimensional Microstructures UsingMicroporous n-Type Silicon’, J. Electrochem. Soc., Vol. 143, 1996. Ageneral review can be found in L. Canham, Ed., Properties of PorousSilicon, INSPEC 1997. Porous silicon is a material that is fabricatedfrom crystalline silicon. Very small pores (nm-micrometer diameters) canbe introduced with a relatively high degree of uniformity and control.

[0089] While reducing the present invention to practice it wasdiscovered that porous silicon, and potentially other materials, havepiezoelectric and piezooptic characteristics. The following embodimentsof the present invention take advantage of the newly discoveredpiezoelectric and piezooptic behavior of porous crystalline materials.

[0090]FIG. 5 shows a piezoelectric device in accordance with theteachings of the present invention, which is referred to hereinbelow asdevice 60. Device 60 includes a first element 62. Element 62 is ofporous crystalline material, such as, but not limited to, poroussilicon. Device 60 further includes a second element 64 which isattached to, or integrally formed with, first element 62. Device 60further includes at least one electrode 66 (three are shown) which is inelectrical contact with first element 62. The arrangement of the abovecomponents is selected such that subjecting first element 62 to anelectric potential via electrode(s) 66 results in a strain induced byfirst element 62 on second element 64. A method of producing apiezoelectric device according to the present invention is effected byattaching to, or integrally forming with, a first element of porouscrystalline material, a second element, and attaching to the firstelement at least one electrode, such that subjecting the first elementto an electric potential via the electrode(s) results in a straininduced by the first element on the second element.

[0091]FIG. 6 shows a piezooptic device in accordance with the teachingsof the present invention, which is referred to hereinbelow as device 70.Device 70 includes a first element 72. Element 72 is of porouscrystalline material, such as, but not limited to, porous silicon.Device 70 further includes a second element 74 which is attached to, orintegrally formed with, first element 72. Device 70 further includes alight source 76, such that subjecting first element 72 to lightoriginating from light source 76 results in a strain induced by firstelement 72 on second element 74. A method of producing a piezoopticdevice according to the present invention is effected by attaching to,or integrally forming with, a first element of porous crystallinematerial, a second element, and providing at least one light source,such that subjecting the first element to light originating from the atleast one light source results in a strain induced by the first elementon the second element.

[0092] According to an additional aspect of the present invention thereis provided a method of inducing strain in an element. The methodaccording to this aspect of the invention is effected by attaching tofirst element, or integrally forming with the element, a second elementof porous crystalline material and subjecting the second element toelectric potential.

[0093]FIG. 7 shows an adaptive reflector in accordance with theteachings of the present invention, which is referred to hereinbelow asreflector 80. Reflector 80 includes a first layer 82 of porouscrystalline material which is attached to, or integrally formed with, asecond layer 84 having a reflective surface 86. Surface 80 may be formedas a reflective coat over first layer 82, or alternatively oradditionally, layer 82 may be polished so as to serve as a reflectingsurface. According to one embodiment of this aspect of the presentinvention reflective surface 86 is designed to reflect light waves,e.g., infrared, visible or ultraviolet light waves, such that reflector80 is an adaptive light reflector, or in other words, an adaptivemirror. According to another embodiment of this aspect of the presentinvention reflective surface 86 is designed to reflect micro waves. Inthis case, a microwave reflecting coat, made of a substance, such as,but not limited to, a metallic layer or a metallic net, is applied ontosurface 86. According to another embodiment of this aspect of thepresent invention reflective surface 86 is designed to reflect radiowaves. In this case, a radio wave reflecting coat, made of a substance,such as, but not limited to, a metallic layer or a metallic net, isapplied onto surface 86.

[0094] According to a presently preferred embodiment of the presentinvention, and as is further shown in FIG. 7, adaptive reflector 80further includes at least one electrode 88, through which an electricpotential is applicable to first layer 82. According to an alternativeembodiment, adaptive reflector 80 further includes at least one lightsource 89 with which light is applicable to first layer 82.

[0095] The structure of the porous crystalline material is important toits piezoelectric/piezooptic response. In low-porosity materials, theresidual matter is not made of separate islands, and thus its conductionis high and the application of voltage to it results in short-circuitingbetween the electrodes (percolation). When the pores capture a largerfraction of the porous matter, it is possible to have an appliedvoltage. The percentage porosity of the percolation limit is differentwith different raw silicon stoichiometry, orientation, processingprocedure and processing chemicals. It will, however, be appreciated inthis respect that these structural parameters of porous crystallinematerials are controllable through their manufacturing methods.

[0096] Additional objects, advantages, and novel features of the presentinvention will become apparent to one ordinarily skilled in the art uponexamination of the following examples, which are not intended to belimiting. Additionally, each of the various embodiments and aspects ofthe present invention as delineated hereinabove and as claimed in theclaims section below finds experimental support in the followingexamples.

EXAMPLES

[0097] Reference is now made to the following examples, which togetherwith the above descriptions, illustrate the invention in a non limitingfashion. While investigating the properties of porous silicon, a newtype of a deformation mechanism was uncovered, transverse electrostaticstrain, by which it is possible to induce strains in an attachedelement, such as membrane mirror.

Example 1

[0098] Construction of a Mirror

[0099] The mirror was constructed of a silicon wafer, polished on bothsides, whose front (mirror) side was evaporated with a reflective layerand the wafer was annealed at 450° C. for approximately 30 minutes. 1″or 2″ n-type wafers were used, with a diameter of the porous area of0.5″ and 1″. The back side of the wafer was made porosive by etching inHF:ehanol (1:1) solution, with the HF itself dissolved in water (1:1).An initial current of 120-130 mA, (24 mA/cm²) was applied to the sample,while it was illuminated by a 50 W halogen lamp. The etching lasted forabout 35 minutes, which sets the thickness of the porous layer to 50micrometers. The sample was washed a few times by ethanol, then left inethanol for 30 minutes for final removal of the HF. It was thenpassivated by growing oxygen on the porous layer, in oxygen atmospherefor 30 minutes at 450° C. This is a standard processing technique ofsilicon wafers, and requires rather a simple equipment. Electrodes werethen made on the porous silicon. As is further exemplified hereinunder,application of voltage between the different electrodes and between theelectrodes and the silicon wafer (whose resistance is negligible) orapplication of light onto the porous silicon lead to deformation of theporous matter, while the unetched side did not deform. As a result, theinduced stress lead to bending of the wafer in a non-local manner,similar to that of a bimorph mirror [Steinhaus E., and S. G. Lipson,‘Bimorph piezoelectric flexible mirror’, J. Opt. Soc. Am. Vol. 69,478-481, 1979].

Example 2

[0100] Theoretical Consideration

[0101] Another way to look at the mirror is to think about it as a setof pistons operating parallel to the surface of the mirror, pulling theelements towards each other and creating bending moments.

[0102] The strain of a plate made of two such layers can be shown tofollow the biharmonic equation, which can further be reduced to theharmonic (Poisson) equation for simple boundary conditions. Since inelectrostatic pull the stress is proportional to the square of thevoltage, the reflected wave front excursions W(r) (equal to twice thestrain) are assumed to follow:

∇² W(r)=γ[dV(r)+mV ²(r)]

[0103] where V(r) is a continuous description of the voltage on theelectrodes. d and m the piezoelectric and electrostrictive constants,and γ depends on geometrical factors and Poisson's ratio and Youngmodulus of the porous silicon and crystal silicon[C. Schwartz, E. Ribak,and S. G. Lipson, ‘Bimorph adaptive mirrors and curvature sensing’, J.Opt. Soc. Am. A Vol. 11, 895-902, 1994]. In this equation, one canmeasure W(r) and V(r), and determine γ separately. This allows one todetermine the constants d and m.

[0104] In order to determine the dependence of strain on voltage, acurvature sensor Tyson[F. Roddier, J. E. Graves, D. McKenna, and M.Northcott, ‘The University of Hawaii adaptive optics system’, SPIE Vol.1542 248-72, 1991; R. K. Tyson, Principles of Adaptive Optics, AcademicPress, 1998] was employed. The intensity distribution when focused onthe sample, and again at a small distance z away from it Vinikman[E. N.Ribak and S. Vinikman, ‘Curvature sensing and intensity transport’, inAdaptive Optics for Insdustry and Medicine, Ed. G. D. Love, WorldScientific, 1999] was measured. The intensity distribution was constantat the sample, and varied as the curvature of the reflected wave frontwhen out of focus. The calculations stems out of the Transport ofIntensity Equation[N Streibl, ‘Phase Imaging by the Transport Equationof Intensity’, Opt. Comm. Vol. 49, 6-10, 1984; M. R. Teague,‘Deterministic Phase Retrieval: A Green's Function Solution’, J. Opt.Soc. Am. Vol. 73, 1434-41, 1983] which relates the intensity along thepath and the phase by:${{k\frac{\partial}{\partial z}{I\left( {r,z} \right)}} = {{- \nabla} \cdot \left\lbrack {{I\left( {r,z} \right)}{\nabla{\phi \left( {r,z} \right)}}} \right\rbrack}},$

[0105] where ∇²=∂²|∂x²+∂²|∂y²; k=2π/λ. For I(x,y,z=0)=I₀=const , thisequation reverts to the curvature equation:${{{{- \frac{k}{I_{0}}}\frac{\partial{I\left( {r,z} \right)}}{\partial z}} = {{\nabla^{2}\phi} + {{\delta (e)}\frac{\partial\phi}{\partial\overset{->}{n}}}}};\quad {e = {edge}}},$

[0106] where {overscore (n)} is the normal vector at the edge of themeasured sample. This is combined with Neumann boundary conditions∂φ/∂{overscore (n)} measured through the intensity around δ(e). One canapproximate the longitudinal derivative as:${\frac{1}{I_{0}}\frac{\partial{I\left( {r,z} \right)}}{\partial z}} \approx \frac{{I\left( {r,{z - {\Delta \quad z}}} \right)} - {I\left( {r,{z + {\Delta \quad z}}} \right)}}{{I_{0} \cdot 2}\Delta \quad z} \approx {\frac{1}{\Delta \quad z}{\frac{{I\left( {r,{z - {\Delta \quad z}}} \right)} - {I\left( {r,{z + {\Delta \quad z}}} \right)}}{{I\left( {r,{z - {\Delta \quad z}}} \right)} + {I\left( {r,{z + {\Delta \quad z}}} \right)}}.}}$

Example 3

[0107] An Optical System Incorporating the Mirror

[0108] Referring now to FIG. 2, a simple optical system 20 wasconstructed that allows one to re-image different planes on a camera 22.System 20 includes a point source 30 and a lens 32 so as to focus lightjust before a surface of a mirror 34. Mirror 34 is so positioned so asto reflect the light through a collimating lens 34 onto a sample 26,which is a mirror in accordance with the teachings of the presentinvention made and constructed as described hereinabove, and from whichthe light is reflected back through lens 34 to a motorized (M) focusinglens 36 which focuses the light onto a CCD camera 22. By moving lens 24in front of camera 22, one can choose two planes to overlap (before andafter reflection from sample 26), or at any two other locations using aframe grabber 40 and an appropriate computer 42 and software.

[0109] In order to find the response of the mirror to voltage, it iseasier to focus one of the locations at the mirror surface itself, andthe second at a distance z off the mirror. Since the sensitivity of themethod depends on the distance from the focus, the first plane will notbe sensitive to phase variations at all, and all the changes will belimited to the second plane. This placement of the planes does notrequire back-and-forth focusing whenever one wishes to take ameasurement: the first intensity measurement does not vary with wavefront changes. If one takes two measurements with and without voltage,and subtract them, the focus measurement will drop out. The modifiedrelationship between the intensities and the displacement is:

I _(v)(r)−I ₀(r)=zI _(F)(r)∇² W(r)

[0110] where I_(F) (r), I_(V) (r) and I₀ (r) are now the intensities atfocus, and off-focus with and without voltage. The equation was solvedwith Dirichlet boundary conditions and the surface profile W(r) deduced.This was repeated for different voltages (FIG. 3) and the value of thecurvature assessed near the electrode (FIG. 4). The piezoelectricconstant was found to be d=5.0·10⁻⁸ m/V and the electrostrictiveconstant m=1.4·10⁻⁹ m/V². At this stage, isotropy of these constants isassumed, although for n-type porous silicon, with preferred direction ofthe pores normal to the layer, this might not be accurate. Most of thenon-quadratic response arises from the voltage difference between thenon-adjacent electrodes, which at first order is a linear correction onthe quadratic voltage dependence between the electrode and the siliconsubstrate.

[0111] The piezoelectric and electrostrictive response of the poroussilicon can be attributed to a number of processes, the foremost ofwhich is electrostatics pull between the porous crystalline materialunder the electrodes. This is supported by the fact that the response isgoverned by the absolute value of the voltage, as in the pull betweencapacitor plates.

[0112] It is also possible that Joule heating of the porous matter is asource for the measured strain or part of it [H. Shinoda, T. Nakjima, K.Ueno, N. Koshida, ‘Thermally induced ultrasonic emission from poroussilicon’, Nature Vol. 400, 6747, 853-855, 1999]. This heating can befound from the knowledge of the power applied to it: it is either theproduct of the current I and voltage V on the sample, or the lightintensity on it. The I-V characteristics are odd (that is, thedependence of power on the sample is different when the voltage ispositive or negative), while the voltage dependence of the strain isnearly even (i.e. independent of the polarization of the voltage).

[0113] Heating of the samples can also arise because of creation ofplasma in the porous volume. Slight variations between the samplesallowed to raise the voltage only up to 50-200 Volts, probably due tobreakdown and plasma creation in the passivated porous matter.Difference between samples arises because the distance between pores andconducting paths inside the silicon are sensitive to the porosity of thesample.

[0114] The time response of the material is rather slow, and it reachesits peak strain only after a minute of applied voltage. This wasmeasured by taking a series of images of the sample as voltage wasapplied to it or as it was illuminated.

[0115] Illumination of the porous silicon with a 6 mW HeNe laser had asimilar but much weaker effect. Without being limited by any theory, itis possible that this piezooptic response is a secondary result ofvoltage developing in the illuminated porous silicon. This raises theoption of controlling the shape of the mirror optically rather thanelectronically.

[0116] Although the invention has been described in conjunction withspecific embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

What is claimed is:
 1. A piezooptic device comprising a first element ofporous crystalline material, a second element being attached to, orintegrally formed with, said first element, and a light source, suchthat subjecting said first element to light originating from said lightsource results in a strain induced by said first element on said secondelement.
 2. The piezooptic device of claim 1, wherein said porouscrystalline material is selected from the group consisting of poroussilicon, or other material with conductive channels and isolatingchannels such as spaces.
 3. The piezooptic device of claim 1, whereinsaid second element is made of a crystalline material.
 4. An adaptivereflector comprising the piezooptic device of claim 1, and a reflectingsurface attached thereto.
 5. The adaptive reflector of claim 4, whereinsaid reflective surface is formed as a reflective coat over said firstlayer.
 6. The adaptive reflector of claim 4, wherein said reflectivesurface is designed to reflect light waves.
 7. The adaptive reflector ofclaim 4, wherein said reflective surface is designed to reflect microwaves.
 8. The adaptive reflector of claim 4, wherein said reflectivesurface is designed to reflect radio waves.
 9. A method of producing apiezooptic device comprising the steps of attaching to, or integrallyforming with, a first element of porous crystalline material, a secondelement, and providing at least one light source, such that subjectingsaid first element to light originating from said at least one lightsource results in a strain induced by said first element on said secondelement.
 10. The method claim 9, wherein said porous crystallinematerial is selected from the group consisting of porous silicon, orother material with conductive channels and isolating channels such asspaces.
 11. The method of claim 9, wherein said second element is madeof a crystalline material.
 12. A method of straining a porouscrystalline material element, the method comprising the step ofsubjecting the porous crystalline material element to light.
 13. Amethod of relaxing a strained porous crystalline material element whichis subjected to light, the method comprising the step of preventing thelight from impinging on the strained porous crystalline materialelement.
 14. A piezooptic device comprising an element of porouscrystalline material and at least one light source being in lightingdistance therefrom, such that subjecting said element to light via saidlight source results in a strain developing in said element.
 15. Amethod of inducing strain in a first element, the method comprising thesteps of attaching to the first element, or integrally forming with thefirst element, a second element of porous crystalline material andsubjecting said second element to light.
 16. The method of claim 15,wherein said porous crystalline material is selected from the groupconsisting of porous silicon, or other material with conductive channelsand isolating channels such as spaces.
 17. The method of claim 15,wherein said second element is made of a crystalline material.